Two-pole step motor for timepiece

ABSTRACT

A stator ( 1 ) is made up by bonding a first stator part ( 1   a ) made of a high-permeability material to a second stator part ( 1   b ) made of a high-permeability material respectively by welding through the intermediary of connections ( 1   c   , 1   d ) made of a low-permeability material or a nonmagnetic material, interposed therebetween. Paired recesses ( 5   a   , 5   b ) serving as holding torque setting means for holding a rotor ( 3 ) by a magnetic action are formed on the inner periphery of a rotor hole ( 2 ) defined inside the stator ( 1 ) at the position forming an initial phase angle (θ 1 ). A field coil ( 7 ) for excitation is magnetically bonded to opposite ends of the stator ( 1 ), thereby forming a two-pole step motor for a timepiece.

This application is a division of Ser. No. 09/719,369 filed Dec. 11,2000 now U.S. Pat. No. 6,548,922 which is a 371 of PCT/JP99/03109 filedJun. 10, 1999.

TECHNICAL FIELD

The invention relates to a two-pole step motor for an analog electronictimepiece.

BACKGROUND TECHNOLOGY

Since an analog electronic timepiece employs a cell for its powersource, the analog electronic timepiece stops its function aftercontinuous operation for a given length of time period due to exhaustionof its capacity. Accordingly, the cell need be replaced periodicallywith a new one, which has been quite troublesome to users.

Further, as the users have to ask specialist shops to do suchreplacement, it has been impossible to have the cell replacedimmediately if the cell runs down when there is a need for use of theanalog electronic timepiece, and consequently, this has caused a greatdeal of inconvenience to the users.

Since such exhaustion of the capacity of the cell of the analogelectronic timepiece poses a major problem to the users, efforts havebeen made lately to study on prolongation of a service life of the cellin the analog electronic timepiece or to develop a timepiece capable ofeliminating a need for replacement of a cell by incorporating agenerator in the timepiece, activated following the motion of the userscarrying the timepiece with them, or by the agency of a power generationmechanism such as a solar cell, and so forth, incorporated in thetimepiece.

However, in the case of an analog electronic timepiece with such a powergeneration mechanism built therein, the timepiece is designed to bedriven by power stored in a capacitor or a secondary cell built therein,however, there have been cases where it has been difficult to generatesufficient power as required all the time because application conditionsof the timepiece varies from one user to another.

Accordingly, even with the timepiece incorporating the power generationmechanism built therein, it has been necessary to aim at achievement oflowering power consumption in order to keep the timepiece in a stableoperational condition without interruption during usage.

Meanwhile, if use can be made of a cell which is large in size, having alarge capacity, it is possible to achieve prolongation of the servicelife thereof, however, designing constraints imposed on a timepiece doesnot permit the cell to be excessively large in size. Accordingly, ifprolongation of the service life of the cell is called for, it has beeninevitable to achieving lowering of power consumption on the part of thetimepiece.

Now, a mechanism of an analog electronic timepiece is broadly describedhereinafter. It has a construction such that a two-pole step motor for atimepiece is intermittently driven in accordance with a reference signalgenerated by a quartz oscillator, and the like, and time display isperformed by transmitting motion of the step motor to the hands of thetimepiece via gears.

It follows therefore that from the viewpoint of power consumption, suchan analog electronic timepiece can be broadly broken down into a circuitpart incorporating the quartz oscillator and the like for generating thereference signal, and a step motor part for rotating the hands of thetimepiece.

However, with analog electronic timepieces in current use, a circuitpart is made up of a semiconductor integrated circuit wherein powerconsumption is rendered small, and consequently, a greater part of poweris after all consumed for driving the step motor for handling the hands.Accordingly, reduction in power consumption of the step motor has aconsiderable effect on lowering of power consumption of a timepiece inwhole.

FIG. 22 is a plan view showing a schematic construction of aconventional two-pole step motor for a timepiece.

The two-pole step motor for a timepiece (referred to hereinafter merelyas “step motor”) comprises a field coil 7 provided with a conductor 7 bwound around a magnetic core 7 a formed of a high-permeability material,and a stator 201 bonded to opposite ends of the magnetic core 7 a of thefield coil 7 by screws 8, 8, respectively, for magnetic connection.

The stator 201 is provided with a rotor hole 202 defined substantiallyat the center thereof, and a rotor 3 is rotatably disposed inside therotor hole 202.

Further, the rotor 3 is comprised of a rotor magnet 3 a and a rotor axle3 b, and the rotor magnet 3 a is made of a ferromagnetic material and isformed in a low-profile columnar shape. The rotor axle 3 b serving as arotation axis is inserted into an axle hole defined at the center of therotor magnet 3 a in the direction normal to the plane of the figure soas to be integrally joined together, thereby magnetizing the rotormagnet 3 a in such a way as to have two poles in the diametricaldirection thereof.

The rotor 3 with opposite ends of the rotor axle 3 b rotatably supportedby bearings (not shown), respectively, is positioned at the center ofthe rotor hole 202. Further, the rotor 3 is constituted such that a gearis provided at one end of the rotor axle 3 b, and rotatory motionthereof is transmitted via the gear to the hands of the timepiece.

Further, holding torque setting means is provided on the inner peripheryof the rotor hole 202, so that the magnetic poles of the rotor magnet 3a are positioned so as to be oriented in a constant direction of aninitial phase angle θ₁ by the agency of the holding torque setting meanswhen the step motor is out of operation, thereby stopping and holdingthe rotor 3 in that position with a predetermined holding torque.

With the step motor, by applying a driving voltage thereto, forward andreverse current are caused to flow alternately through the field coil 7,thereby a magnetic field oriented in a direction corresponding to thedirection of the forward and reverse current, respectively, is generatedinside the rotor hole 202 so as to correspond to the magnitude of therespective flowing current, and the magnetic field is caused to act onthe rotor magnet 3 a magnetized beforehand, so that the rotor 3 isrotated by 180 degrees (for one step) counterclockwise in FIG. 22.

The motion of the step motor, made for one step, is describedhereinafter.

If the direction of a magnetic field produced inside the rotor hole 202by magnetic fluxes which are generated when current is caused to flowthrough the field coil 7 is designated as an excitation direction line12, the rotor 3 is held and stopped at a position where a line 4, whichis the direction of magnetization of the rotor magnet 3 a, and whichinterconnects the two poles thereof, is rotated by the initial phaseangle θ₁ counterclockwise in FIG. 22, relative to the excitationdirection line 12, by the agency of the holding torque of the holdingtorque setting means, established by magnetic action between themagnetic poles of the rotor magnet 3 a and the stator 201 in a statewhere no current flows through the field coil 7.

In this state, when current is caused to flow through the field coil 7in such a direction as to cause the rotor 3 to rotate forward, magneticfluxes occur to the field coil 7, and a magnetic field is generatedinside the rotor hole 202, whereupon the rotor 3 is subjected to arotational torque caused by an interaction of the magnetic field and thepermanent magnetized charge of the rotor magnet 3 a, starting rotationagainst the resistance of the holding torque. Upon flowing of currentthrough the field coil 7 for a suitable duration only, the rotor 3 stopsafter being rotated through 180° up to a position of the next stop.

With the step motor of the constitution as described above, powerconsumption for a unit of time is expressed by the product of a strengthof the current caused to flow through the filed coil 7 for excitation,and a cell voltage as applied. Since the cell voltage as applied in thiscase remains substantially constant, lowering of the power consumptionof the step motor depends on how to reduce current flowing in the fieldcoil 7 while satisfying driving characteristics required of the stepmotor.

Further, with the step motor, the rotational torque is caused to occurto the rotor 3 by causing current to flow in the field coil 7, therebycausing the rotor 3 to rotate against the resistance of the holdingtorque. Consequently, the smaller the holding torque, the smaller therotational torque as required may be in proportion to the holdingtorque.

Since current which is caused to flow in the field coil 7 isproportional to the rotational torque, current flowing in the field coil7 can be reduced if the holding torque can be reduced. As a result, itbecomes possible to achieve lowering of power consumption of the stepmotor for a timepiece.

Now, the holding torque of the step motor for a timepiece has functionssuch that even when the timepiece is subjected to an impact when thetimepiece is dropped, and so forth, the hands are securely held so asnot to be caused to jump, thereby enabling correct time to be displayedwhile settling the hands at a correct stop position against theresistance of friction torque occurring to bearings and gears inside thetimepiece.

Accordingly, it is not as simple as a case where the holding torque needonly be rendered smaller in order to reduce power consumption, but it isrequired that the holding torque be set so as to meet the minimumholding torque as required to maintain the function of the timepiece.

As disclosed in International Publication No. WO 98/30939, it isdescribed with reference to the holding torque as required for use intimepiece that jumping of the hands will not occur if kinetic energyoccurring to the hands by an impact is smaller than a holding potentialestablished by the holding torque of a rotor, that is, a magneticpotential difference.

Since kinetic energy received by the hands when subjected to the impactis proportional to the square of moment of the hands, the holdingpotential, that is, the holding torque can be rendered smaller by use ofthe hands with a smaller moment.

By so doing, it becomes possible to set the minimum holding torque asrequired at a very small value equivalent to a fraction of the holdingtorque of a step motor for a timepiece, thereby achieving lowering ofpower consumption of the timepiece.

Next, holding torque setting means, provided in a stator of theconventional step motor for a timepiece, is now described hereinafter.

As for the holding torque setting means, provided in the stator of theconventional step motor for a timepiece, there are primarily two typesin construction as described below.

One type has a construction such that the stator 201 of the step motorfor a timepiece , shown in FIG. 22, is formed of a high-permeabilitymaterial, and as shown in FIG. 23, there are provided holes 6, 6 definedclose to opposite ends of the stator 201 in the longitudinal direction,for bonding the stator to opposite ends of the magnetic core 7 a of thefield coil 7.

A rotor hole 202 provided substantially at the center of the stator 201is defined in the shape of two semicircles joined together with thecenter of the respective semicircles deviated from each other to permita holding torque and an initial phase angle θ₁ (refer to FIG. 22) to beset.

By combining the two semicircles in such a way as to cause the center ofthe respective semicircles to deviate from each other, two stepped parts204 a, 204 b having a gap amount G, respectively, are formed. With thestator 201, it is possible to set the holding torque to a desired valueby adjusting the gap amount G.

The construction wherein such stepped parts 204 a, 204 b described aboveare formed inside the rotor hole 202 of the stator 201 is described in,for example, Japanese Patent Laid-open No. S 49-132507.

A stator wherein such stepped parts are formed inside a rotor holethereof is hereinafter referred to as a gap type stator.

Next, the construction of another type of holding torque setting meansis described hereinafter with reference to FIG. 24. In the figure, somecomponents used in common is described where necessary by using the samereference numerals as described with reference to FIG. 22.

A stator 211 in this case is provided with a pair of recesses 205 a, 205b formed at symmetrical positions against the center axis of the rotorhole 212 on the inner periphery of the rotor hole 212, as holding torquesetting means in order to provide the holding torque and the initialphase angle of a rotor 3.

Further, a straight line 24 passing through the respective centers ofthe recesses 205 a, 205 b is disposed so as to be tilted at an angle ofθ₁₁, relative to an excitation direction 21 of the rotor hole 212.

With the stator 211, an angle which the straight line 24 passing throughthe respective centers of the recesses 205 a, 205 b forms with astraight line 27 passing through the center axis of the rotor hole 212and orthogonal to the excitation direction of the stator 211, isdesignated as an installation angle θ₁₂ of the recesses 205 a, 205 bexpressed in a positive value when rotated in counterclockwisedirection, and the initial phase angle θ₁ (refer to FIG. 22) of therotor 3 is set by adjusting the installation angle θ₁₂.

In the case of the step motor for a timepiece, having the stator 211 ofsuch a construction as described above, the holding torque of the rotor3 is determined by the pair of the recesses 205 a, 205 b.

In this connection, the construction wherein the recesses 205 a, 205 bas described above are formed inside of the rotor hole 212 of the stator211 is described in, for example, Japanese Patent Laid-open No. S51-1908.

A stator wherein recesses are formed inside a rotor hole thereof ishereinafter referred to as a notched type stator.

As described in the foregoing, in the case of the conventional stepmotor employing the gap type stator, the magnitude of the holding torqueand the initial phase angle can be adjusted by varying the gap amount ofthe stepped parts formed inside the rotor hole.

With the ordinary step motor for a timepiece, since the diameter of therotor hole is in the order of 1700 μm on average, the maximum holdingtorque can be set to around 300 nNm by setting the gap amount of thestepped parts of a stator to about 40 to 50 μm.

However, if it is intended to further reduce the holding torque to alarge extent in order to achieve lowering of power consumption, the gapamount need be rendered to be extremely small, as small as about 10 μm,and consequently, it becomes difficult in respect of precision withwhich to process the stator to establish a stable holding torque.

Further, if the gap amount is rendered to be extremely small asdescribed above, this leads to resultant reduction in the initial phaseangle (refer to θ₁ in FIG. 22). As a result, this will result inrequirement for large power consumption when driving the rotor, so thatlowering of power consumption can not be achieved.

Further, in the case of the step motor employing the gap type statorconstruction, it is possible to set the holding torque to a small valueeven at the same gap amount without varying the initial phase angle byenlarging the diameter of the rotor hole, however, such enlargement ofthe diameter of the rotor hole will result in reduction of interactionbetween a magnetic field occurring inside the rotor hole and the rotormagnet.

That is, in this case, as electromechanical coupling constant decreases,the rotational torque occurring to the rotor by flow of current throughthe field coil is reduced.

As a result, even if the initial phase angle is set to a proper value bylowering the holding potential established by the holding torque, itwill become necessary to increase current flowing in the field coil tocompensate for a decrease in the rotational torque due to a decrease ofthe electromechanical coupling constant, so that a power-saving effectresulting from the holding torque being set to a small value will beoffset, thereby rendering it impossible to achieve lowering of powerconsumption.

Meanwhile, in the case of the step motor employing the notched typestator construction, the initial phase angle can be set by theinstallation angle of the pair of the recesses while the holding torqueis adjusted by either increasing or decreasing the sum of areas of therecesses formed on the inner periphery of the rotor hole, andconsequently, if it is intended to render the holding torqueconsiderably less than the present value in order to lower powerconsumption, this will require the sum of the areas of the pair of therecesses, in other words, dimensions of the recesses to be renderedextremely small. Accordingly, it will become difficult in respect ofprecision with which to process the stator to obtain a stable holdingtorque.

Further, with the notched type stator as well, it is possible to set theholding torque to a small value without varying the sum of the areas ofthe recesses by enlarging the diameter of the rotor hole, however, aswith the case of the gap type stator, such enlargement of the diameterof the rotor hole will result in a decrease of the electromechanicalcoupling constant, so that lowering of power consumption can not beachieved.

As described hereinbefore, with the stator of the conventionalconstruction as described above, if it is intended to set the holdingtorque to a small value in an attempt to further reduce powerconsumption, it has been necessary to render either the gap amount ofthe stepped parts formed in the stator or the dimensions of the recessesformed in the stator to be extremely small, thus posing difficulty inrespect of precision with which to process the stator. Accordingly, ithas been difficult to set a stable holding torque.

Consequently, with the step motor for a timepiece, adopting theconventional construction, it has been difficult to achieve lowering ofpower consumption.

DISCLOSURE OF THE INVENTION

The invention has been developed against the technical backgrounddescribed above, and it is an object of the invention to solve theproblems as described above by devising a novel construction of astator, and to provide a two-pole step motor for a timepiece which issuitable for lowering of power consumption, and which can bemanufactured with ease.

To achieve the above objects, a two-pole step motor for a timepieceaccording to the invention comprises: a rotor made up of a rotor magnetand a rotor axle; a stator made of a high-permeability material, havinga rotor hole in which the rotor is installed; and a field coil forexcitation, provided with a magnetic core made of a high-permeabilitymaterial around which a conductor is wound, and opposite ends of whichare magnetically bonded to opposite ends of the stator, wherein thestator is provided with a plurality of holding torque setting means,disposed on the inner periphery of the rotor hole, at installationangles differing in the direction of the inner periphery.

Herein, the installation angle of the holding torque setting meansrefers to an installation angle relative to the direction orthogonal toan excitation direction of the stator, and the installation angle thatdiffers by 180° is deemed to be an equivalent installation angle.

It is effective for attaining lowering of power consumption to set theinitial phase angle θ₁ which is an angle formed by magnetic fielddirection line in the direction of a magnetic field produced inside therotor hole and the magnetizing direction line of the rotor magnet at thestandstill position of the rotor based on respective installation anglesof the plurality of the holding torque setting means, in a range of 50degress to 70 degress.

With the two-pole step motor for a timepiece according to the invention,even in the case where a single holding torque setting means can not beinstalled at an installation angle required for obtaining an initialphase angle and a holding torque as intended owing to presence of theaxle hole of gears or holes of fixed pins which are formed around therotor hole of the stator, the initial phase angle and the holding torqueas intended can be obtained by breaking down a holding torqueestablished by a pair of the holding torque setting means into vectors,and by installing two or more holding torque setting means correspondingto the respective vectors as broken down, at different installationangles and at locations avoiding the axle hole and the holes of fixedpins.

Further, the stator is preferably made up by bonding a first stator partmade of a high-permeability material to a second stator part made of ahigh-permeability material through the intermediary of connections madeof a low-permeability material or a nonmagnetic material.

In such a case, the stator has a construction such that it ismagnetically separated into two portions, and consequently, a magneticfield inside the rotor hole for rotating the rotor can be efficientlyproduced by magnetic fluxes excited by the field coil, so that currentcaused to flow in the field coil can be reduced, thereby attaininglowering of power consumption.

Further, since the connections are made of either a low-permeabilitymaterial or nonmagnetic material, there is no need of narrowing down theconnections to an extreme extent, thereby enabling mechanical strengthas required to be secured.

Furthermore, with the two-pole step motor for a timepiece describedabove, it is preferably that the connections where the first stator partis bonded to the second stator part serve as at least one of theplurality of the holding torque setting means while other holding torquesetting means except the connections are disposed on the inner peripheryof the rotor hole at installation angles differing from that for theconnections.

In addition, the plurality of the holding torque setting means arepreferably paired recesses or paired protuberances formed on the innerperiphery of the rotor hole, respectively. Further, the other holdingtorque setting means as described above are preferably a pair ofrecesses or a pair of protuberances formed on the inner periphery of therotor hole, including means formed in a shape asymmetrical with respectto the center of the rotor hole.

Then, the holding torque can be adjusted by varying the dimensions ofthe recesses or the protuberances, and the initial phase angle can beadjusted by varying the installation position of the recesses or theprotuberances.

Further, among the holding torque setting means, the means formed in theshape asymmetrical with respect to the center of the rotor hole may be apair consisting of a recess and a protuberance facing each other, formedon the inner periphery of the rotor hole, on opposite sides of thecenter of the rotor hole, or may comprise a recess or a protuberanceformed on the inner periphery of the rotor hole only on one side of thecenter thereof.

Furthermore, the plurality of the holding torque setting means arepreferably combination of those of different types with the installationangles thereof in the direction of the inner periphery of the rotor holediffering from each other.

In this connection, the combination of those of different types amongthe holding torque setting means is preferably combination of the gaptype and the notched type, described in the foregoing, or combination ofan oval type as described hereinafter and the notched type describedabove.

Still further, with the two-pole step motor for a timepiece having theplurality of the holding torque setting means including the means formedin a shape asymmetrical with respect to the center of the rotor hole,the first stator part is preferably bonded to the second stator partthrough the intermediary of the connections made of a low-permeabilitymaterial or nonmagnetic material, the connections serving as at leastone of the plurality of the holding torque setting means.

Similarly, with the two-pole step motor for a timepiece having theplurality of the holding torque setting means of different types, thefirst stator part is preferably bonded to the second stator part throughthe intermediary of the connections made of a low-permeability materialor nonmagnetic material, the connections serving as at least one of theplurality of the holding torque setting means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a schematic construction of a two-polestep motor for a timepiece according to a first embodiment of theinvention.

FIG. 2 is a plan view showing the construction of a stator of thetwo-pole step motor for a timepiece in FIG. 1.

FIG. 3 is a graph showing the relation between a holding torque of thetwo-pole step motor for a timepiece in FIG. 1 and a power consumption ofthe same.

FIG. 4 is a graph showing the relation between an initial phase angle ofthe two-pole step motor for a timepiece in FIG. 1 and a powerconsumption of the same.

FIG. 5 is a plan view showing a schematic construction of a two-polestep motor for a timepiece according to a second embodiment of theinvention like FIG. 1.

FIG. 6 is a plan view showing the construction of a stator of thetwo-pole step motor for a timepiece in FIG. 5.

FIG. 7 is a plan view enlarging a recess provided in the stator servingas holding torque setting means in FIG. 5.

FIG. 8 is a graph showing the relation between installation angles of apair of recesses 15 e, 15 f of the two-pole step motor for a timepiecein FIG. 5, a maximum holding torque and initial phase angle of the same.

FIG. 9 is a graph showing the relation between each depth of the recessand the maximum holding torque of the two-pole step motor for atimepiece in FIG. 5.

FIG. 10 is a graph showing the relation between each width of the recessand the maximum holding torque of the step motor for a timepiece in FIG.5.

FIG. 11 is a plan view showing a schematic construction of a step motorfor a timepiece according to a third embodiment of the invention likeFIG. 1.

FIG. 12 is a plan view showing the construction of a stator of the stepmotor for a timepiece in FIG. 11.

FIG. 13 is a plan view showing a schematic construction of a step motorfor a timepiece according to a fourth embodiment of the invention likeFIG. 6.

FIG. 14 is a plan view showing the construction of a stator of the stepmotor for a timepiece according to a fifth embodiment of the inventionlike FIG. 12.

FIG. 15 is a plan view showing the construction of a stator of the stepmotor for a timepiece according to a sixth embodiment of the inventionlike FIG. 13.

FIG. 16 is a plan view showing the construction of a stator of the stepmotor for a timepiece according to a seventh embodiment of the inventionlike FIG. 15.

FIG. 17 is a plan view showing the construction of a stator of the stepmotor for a timepiece according to an eighth embodiment of the inventionlike FIG. 14.

FIG. 18 is a plan view showing the construction of a stator of the stepmotor for a timepiece according to a ninth embodiment of the inventionlike FIG. 15.

FIG. 19 is a plan view showing the construction of a stator of the stepmotor for a timepiece according to a tenth embodiment of the inventionlike FIG. 17.

FIG. 20 is a plan view showing the construction of a stator of the stepmotor for a timepiece according to an eleventh embodiment of theinvention like FIG. 18.

FIG. 21 is a plan view showing the construction of a stator of the stepmotor for a timepiece according to a twelfth embodiment of the inventionlike FIG. 20.

FIG. 22 is a plan view showing a schematic construction of aconventional two-pole step motor for a timepiece.

FIG. 23 is a plan view showing the construction of a stator of the stepmotor for a timepiece like FIG. 2.

FIG. 24 is a plan view showing the construction of a stator having apair of recesses serving as conventional holding torque setting meanslike FIG. 23.

FIG. 25 is a plan view showing another construction of a stator of aconventional step motor for a timepiece like FIG. 24.

FIG. 26 is a plan view showing just a protuberance in which theprotuberance is shaped like the recesses shown in FIG. 13, but faces theopposite direction.

BEST MODE FOR CARRYING THE INVENTION

The embodiments of the inventions are now described in detail withreference to the accompanied drawings.

First Embodiment: FIGS. 1 to 4

FIG. 1 is a plan view showing a schematic construction of a two-polestep motor for a timepiece according to a first embodiment of theinvention, FIG. 2 is a plan view showing the construction of a stator ofthe two-pole step motor for a timepiece in FIG. 1, FIG. 3 is a graphshowing the relation between a holding torque of the two-pole step motorfor a timepiece in FIG. 1 and a power consumption of the same, and FIG.4 is a graph showing the relation between an initial phase angle of thetwo-pole step motor for a timepiece in FIG. 1 and a power consumption ofthe same.

The two-pole step motor for a timepiece (hereinafter referred to simplyas a step motor) comprises: a rotor 3 made up of a rotor magnet 3 a anda rotor axle 3 b; a stator 1 having a rotor hole 2 in which the rotor isinstalled; and a field coil 7 for excitation, provided with a magneticcore 7 a made of a high-permeability material around which a conductor 7b is wound, and opposite ends of which are magnetically bonded toopposite ends of the stator 1.

Inasmuch as the construction of the step motor is the same as that ofthe conventional two-pole step motor for a timepiece as explained withreference to FIG. 22, components which are the same as those shown inFIG. 22 are depicted by the same reference numerals, and the explanationthereof is omitted.

The stator 1 of this step motor comprises a first stator part 1 a and asecond stator part 1 b formed of a high-permeability materialrespectively (hereinafter simply referred to as stator part 1 a andstator part 1 b) which are bonded to each other through the intermediaryof connections 1 c, 1 d formed of a low-permeability material ornonmagnetic material respectively.

A plurality of holding torque setting means are provided at the innerperiphery of the rotor hole 2 defined substantially at the center of thestator 1 for holding the rotor 3 at a given position of the rotatingdirection of the rotor 3 as shown in FIG. 1, namely, the position wherea line 4 for connecting two magnetic poles which are magnetized in thediametrical direction of the rotor magnet 3 a is positioned at an angleof initial phase angle θ₁ when the step motor is not driven so that therotor 3 is not rotated, with a given holding torque owing to a magneticaction between the magnetic poles of the rotor magnet 3 a and the stator1.

The plurality of holding torque setting means comprise a pair ofrecesses 5 a, 5 b formed in the inner periphery of the rotor hole 2 anda pair of connections 1 c, 1 d wherein both pairs are positioned at thesymmetrical positions with respect to the center of the rotor hole 2, asshown in FIG. 2.

The pair of recesses 5 a, 5 b and the pair of connections 1 c, 1 d aredifferent from each other in installation angles as shown in thedrawing. The installation angle is an arrangement angle relative to aline orthogonal to a magnetic field direction line 12 in an excitationdirection of the stator 1, however, the detail thereof is explained withreference to a second embodiment and subsequent embodiments of theinvention (FIG. 6 and subsequent figures).

Holes 6, 6 are formed on both end portions of the stator 1 in thelongitudinal direction so as to connect magnetically to both terminalsof the magnetic core 7 a of the field coil 7.

In order to manufacture this stator 1, a pilot hole being thepositioning hole for the later press working, a prepared hole for therotor hole 2, and fixing holes 6, 6 are formed by pressing a highpermeability band permalloy of 500 μm thickness, and the external shapepartially remaining a joint part (not illustrated) to couple the bandmaterial is punched.

Next, slits of 200 μm width are punched on the connections 1 c, 1 d,wires of predetermined length made of a low permeability ornon-permeability material are inserted in the slits, and the stator part1 a and the stator part 1 b separated by the slits are joined to bematched by the laser welding.

Then, the rotor hole 2 and the recesses 5 a, 5 b are punched by pressworking, and finally the joint part to couple the band material ispunched to complete the external shape working. And, the magneticannealing is applied to the above band material that completed theexternal shape working to make up the stator 1 of the step motor.

Described next is a result of an experiment which is performed fordetermining a proper condition necessary for achieving lowering of powerconsumption when forming the step motor into which the stator 1 havingthe foregoing construction is integrated.

With the experiment, the relation between a holding torque of the stepmotor and the power consumption, the relation between an initial phaseangle and a power consumption, and the relation between a powerconsumption and the construction of the connections of the step motorwere checked.

The experiment was performed by the known two-pole step motor for atimepiece into which the stator 1 according to the first embodiment ofthe invention was integrated so as to perform measurement. A chopperdriving waveform is used as a driving waveform and an ON/OFF ratio ofeach pulse of the driving waveform is adjusted, and determined theminimum consumption power capable of performing a normal driving.

Particularly, regarding the stator 1 that was used for measurement, theportion of the recesses 5 a, 5 b are not formed by press working butthey are formed by an electric discharge machining in accordance withvarieties of measurement conditions before a magnetic annealing isapplied thereto.

First of all, the relation between the holding torque and the powerconsumption is described.

The power consumption in the step motor for a timepiece can be probablyachieved by reducing the holding torque because a current necessary forexcitation caused to flow in the field coil 7 can be reduced by reducingthe holding torque.

Accordingly, the change of the power consumption when the holding torquewas changed actually was checked with experiments.

With regard to the stator 1 used for the measurements, several kindshaving different cuts of depths of the semicircular recesses 5 a, 5 bprovided on the inner periphery of the rotor 2 were prepared to adjustthe holding torque. The self-made measuring instrument of the rotorrotational angle measured the angular velocity against the displacementangle of the rotor 3, and from the measured results and the inertia ofthe rotor 3, the equation of motion was solved, thereby calculating theholding torques as to the stator 1.

And, the power consumption was calculated through integrating theproducts of a current caused to flow through the coil 7 and a drivevoltage applied across it when the motor was driven for one step.Further, since the magnitude of the holding torque depends upon thedisplacement angle, the comparison of the measurement was made using themaximum holding torque. And, for the maximum holding torque of thestator 1 used for the measurements, the torques of 50 nNm through 250nNm were prepared in increments of about 50 nNm.

The measurement result by this experiment is shown in FIG. 3. Accordingto the measurement result, when the maximum holding torque is 250 nNm,the power consumption is about 800 nJ, while when the maximum holdingtorque is lowered to 100 nNm, the power consumption becomes about 350nJ, from which it has been found that the relation between the setmaximum holding torque and the power consumption necessary for drivingfor one step is substantially proportional.

In such a manner, since when the holding torque is reduced to half, thepower consumption is also reduced to half, so that it has been foundthat the reduction of the holding torque has a great effect on thereduction of the power consumption.

Meanwhile, as mentioned above, in the step motor for a timepiece, theminimum holding torque is required to prevent the hands from jumping byan impact when the timepiece is dropped, or to indicate an exact time,and to enable hands to be stably stopped at the standstill positionagainst the resistance of frictional torque produced at the bearing andgears.

If a motion energy which hands of a timepiece receive due to the impactis smaller than the holding potential formed by the holding torque ofthe rotor, the jumping of the hands do not occur. That is, when themoment of hands is adjusted to be small, the holding potential can bemade small, the holding torque can be made small.

In such a manner, when the moment of hands is adjusted to be small, theminimum holding torque as required can be reduced to an extremely smallholding torque, namely, to a fraction of the holding torque of theordinary step motor for a timepiece.

As mentioned above, it has been found, from the measurement result shownin FIG. 3, that the reduction of the holding torque is effective for thereduction of the power consumption, and this holding torque can bereduced to an extremely small value, namely, at a fraction of a holdingtoque of an ordinary step motor for a timepiece.

Then, the relation between the initial phase angle and the powerconsumption is described.

The initial phase angle θ₁ shown in FIG. 1 shows a phase differencebetween the rotary torque and the holding torque, and it is an angleformed by the magnetic field direction line 12 in the direction of amagnetic field produced inside the rotor hole 2 and the magnetizingdirection line 4 of the rotor magnet 3 a at the standstill position ofthe rotor 3, which is a very important parameter for driving the stepmotor.

With the experiment, several kinds of stators 1 are used, which are setthe maximum holding torque to a fixed value of 150 nNm and changed thesetting positions of the recesses 5 a, 5 b formed on the inner peripheryof the rotor hole 2, thereby changing only the initial phase angle so asto check the relation between the initial phase angle and the powerconsumption.

Further, the initial phase angle is calculated by measuring the angularvelocity relative to the displacement angle of the rotor 3 by aself-prepared rotor rotary angle measuring device and also by measuringa counter-electromotive current produced in the field coil 7 when therotor 3 performs a rotary motion.

The power consumption is calculated by integrating the product of thecurrent caused to flow through the field coil 7 and a drive voltageapplied across it when the step motor is driven for one step. Theinitial phase angle of the stator 1 used in the measurement ranges from20 degrees to 80 degrees in increments of substantially 10 degrees.

A measurement result by the experiment is shown in FIG. 4. According tothe measurement result, the power consumption is reduced sharply whenthe initial phase angle ranges from 20 degrees to 40 degrees and itbecomes the minimum when the initial phase angle ranges from 50 degreesto 60 degrees, and it is increased gently when the initial phase angleis increased thereafter.

Generally speaking, although 45 degrees are used as the optimum value ofthe initial phase angle, it has been found from this experiment wherethe maximum holding torque is reduced to the small value such as 150 nNmthat the optimum value of the initial phase angle where the powerconsumption becomes the minimum is transferred to the side of an anglethat is larger than 45 degrees. From the foregoing result, it has beenfound that the achievement of lowering power consumption of the stepmotor for a timepiece requires the setting of the optimum initial phaseangle. It has been found, when the maximum holding torque is reduced toa small value such as 150 nNm, the optimum initial phase angle becomeslarger than 45 degrees and the same angle becomes 50 to 60 degreesaccording to the present measurement result.

Then, the relation between the power consumption and the construction ofthe connections of the stator is now described.

In the case of the construction of the stator 1 as shown in FIG. 2,where the stator part 1 a and stator part 1 b of the stator 1 areconnected to each other by connections 1 c, 1 d that are made of alow-permeability material or nonmagnetic material, it is possible toseparate the stator 1 into the stator part 1 a and the stator part 1 bso that a magnetic field necessary for rotating the rotor 3 can beproduced efficiently inside the rotor hole 2.

Accordingly, an experiment for checking the relation between theconnections and the power consumption is performed.

With this experiment, a case using the stator 1 shown in FIG. 2 iscompared with a case using the stator 221 having a pair of recesses 5 a,5 b used in the conventional step motor for a timepiece shown in FIG.25.

In the case of using the conventional stator 221, two kinds of statorsare prepared, namely, one having a connection 223 a, 223 b made of ahigh-permeability material with the minimum width B of 100 μm andanother having a connection 223 a, 223 b made of the same material withthe minimum width B of 200 μm.

On the other hand, the stator 1 having the construction as explainedwith reference to FIG. 2 has each width Wa between both ends of theconnections 1 c, 1 d is 300 μm, respectively, while each interval Wbbetween the stator part 1 a and stator part 1 b is 200 μm, respectively.

Suppose that both the stators 1 and 221 to be compared with each otherhave the same holding torque and the initial phase angle.

It has been found as the result of the experiment that the rotor can notbe rotated in the case of using the conventional stator 221 having theconnections 223 a, 223 b each having minimum width B of 200 μm because asufficient magnetic field can not be produced inside the rotor hole 222by the employed driving waveform.

On the other hand, it has been observed that the rotor can be rotated inthe case of using the stator 221 that is prone to produce a magneticsaturation by making the minimum width B of each interval of theconnections 223 a, 223 b being 100 μm.

As a result of comparison between a power consumption at that time andthat when using the stator 1, it has been observed that the powerconsumption when using the stator 1 can be reduced by about 20% comparedwith the case using the stator 221.

Further, since the connections 1 c, 1 d of the stator 1 are made of alow-permeability or nonmagnetic material, the connections are notnecessary to be extremely small, so that a sufficient mechanicalstrength can be secured.

As a result of the experiment, it has been observed that the stator 1 ispreferable to have the connections 1 c, 1 d made of a low-permeabilitymaterial or nonmagnetic material in view of the achievement of loweringof power consumption.

In view of the result of checking the relation between the holdingtorque of the step motor for a timepiece and power consumption, therelation between the initial phase angle and the power consumption, andthe relation between the power consumption and the structure of theconnections, it has been observed that there are following threeconditions necessary for realizing the lowering power consumption of thestep motor for a timepiece.

That is, the first is that the holding torque is to be the minimum asrequired. The second is that an optimum initial phase angle can be setindependently of a holding torque. The third is that the stator has aconstruction to be divided magnetically into two portions at the rightand left sides.

Accordingly, if a stator that satisfies the foregoing three conditionsat the same time is used, the intended lowering of power consumption ofthe step motor for a timepiece can be achieved.

That is, more in detail, as shown in FIG. 2, it is most preferable tostructure the stator 1 such that a plurality of recesses 5 a, 5 b areformed on the inner periphery of the rotor hole 2 so as to set theholding torque and the initial phase angle θ₁ independently from eachother, and the two divided stator part 1 a and stator part 1 b arebonded to each other by the connections 1 c, 1 d made of alow-permeability or nonmagnetic material.

If the stator 1 has such a construction, the holding torque can beadjusted depending on the sizes of the recesses 5 a, 5 b and the initialphase angle can be adjusted by changing the setting positions of therecesses 5 a, 5 b, so that the holding torque adapted for the achievinglowering of power consumption and the initial phase angle can be setindependently of each other.

The step motor embodying the foregoing construction was actuallymanufactured, and a power consumption thereof was measured, then theeffect relating to the lowering of power consumption was checked, whichis described hereinafter.

With the experiment, a power consumption of the known two-pole stepmotor for a timepiece incorporated therein the stator 1 of the firstembodiment of the invention instead of the conventional stator ismeasured.

The stator 1 used in the experiment employs a permalloy having athickness of 500 μm as a material, and the connections 1 c, 1 d employsNichrome. The dimensions of the connections 1 c, 1 d are set to 300 μmin width Wa while an interval Wb between the stator part 1 a and statorpart 1 b shown in FIG. 2 is set to 200 μm.

Further, the diameter of the rotor hole 2 formed in the stator 1 is setto 1300 μm, and the initial phase angle is set to 60 degrees byadjusting the sizes and setting positions of the recesses 5 a, 5 bformed on the inner periphery of the rotor hole 2, while the holdingpotential is set to 100 nNm. A samarium cobalt magnet having an outerdiameter of 800 μm and a thickness of 400 μm is used as the rotor magnet3 a.

When a power consumption of the step motor for a timepiece in which thestator 1 is incorporated is checked, it was possible to achieve a powerconsumption of about 300 nJ for one step.

As a result, a power consumption was reduced by about 20% compared withthe two-pole step motor for a timepiece incorporated therein theconventional stator 221 as explained with reference to FIG. 25.

Second Embodiment: FIGS. 5 to 10

A two-pole step motor for a timepiece according to a second embodimentof the invention is now described with reference to FIGS. 5 to 10.

FIG. 5 is a plan view of a two-pole step motor for a timepiece accordingto a second embodiment of the invention like FIG. 1, FIG. 6 is a planview showing the construction of a stator of the two-pole step motor fora timepiece in FIG. 5 and FIG. 7 is a plan view enlarging a recessprovided in the stator and serving as holding torque setting means.

The two-pole step motor for a timepiece according to the secondembodiment is the same as that of the first embodiment of the inventionas explained with reference to FIGS. 1 to 4 except the construction of astator, and hence the explanation for the same components are omitted.

The two-pole step motor for a timepiece according to the secondembodiment as shown in FIG. 5 has a stator 11 having the shape as shownin FIG. 6, and two pairs of recesses, i.e. one pair of recesses 15 c, 15d and another pair of recesses 15 e, 15 f serving as a plurality ofholding torque setting means are formed on the inner periphery of therotor hole 2 at the installation angles which are different from eachother in the peripheral direction.

One pair of recesses 15 c, 15 d and another pair of recesses 15 e, 15 fare formed at the positions which are symmetrical with respect to thecenter of the rotor hole 2.

The stator 11 is made of a high-permeability material, and one pair ofrecesses 15 c, 15 d and another pair of recesses 15 e, 15 f are formedrespectively to set a holding torque and an initial phase angle θ₁, andan angle formed by a straight line 25 passing through centers of therecesses 15 c, 15 d and a straight line 27 passing through the center ofthe rotor hole 2 and orthogonal to a magnetic field directed line 12 inan excitation direction of the stator 1 is defined as an installationangle θ₂ of the recesses 15 c, 15 d.

Further, an angle formed by a straight line passing through the centersof another pair of recesses 15 e, 15 f (it conforms to a magnetic fileddirected line 12 in FIG. 6) and the line 27 is defined as aninstallation angle θ₃ of recesses 15 e, 15 f. Both the installationangles θ₂, θ₃ define a positive value in a counterclockwise direction inFIG. 6.

If the installation angles between two holding torque setting means aredifferent from each other by 180 degrees relative to the peripheraldirection of the rotor hole 2, they are considered to be the sameinstallation angles. The reason is that if the phase difference of theinstallation angles of the two-pole step motor is 180 degrees, anelectric phase difference becomes 360 degrees which is double theinstallation angles, and hence the same directed torque operates in thetwo holding torque setting means.

All the respective recesses 15 c to 15 f have the same sizes and shapes,wherein the recess 15 f as shown in an enlarged view of FIG. 7 has asubstantially rectangular shape formed by cutting the portion encircledbetween the inner diameter of the rotor hole 2 and a circular arc 16 ofa circle that is concentric the rotor hole 2, and each width of the cutor notched portions is defined as a width Wc. Each depth D of therespective recesses 15 c to 15 f is the difference between the radius R1of the rotor hole 2 and a radius R2 of the circular arc 16.

When the stator 11 is manufactured, it is formed of a band materialhaving a thickness of 500 μm and made of permalloy which is ahigh-permeability material, and press working is applied to the bandmaterial to form a pilot hole being the positioning hole for the pressworking, a prepared hole for the rotor hole 2, and fixing holes 6, 6,and the external shape partially remaining a joint part (notillustrated) to couple the band material is punched.

Thereafter, the rotor hole 2 and the parts of the recesses 15 c, 15 dand recesses 15 e, 15 f are punched by press working, and finally theportion to be coupled to the band material is punched, therebycompleting the external shape working of the stator 11. Thereafter, themagnetic annealing is applied to the band material to make up the stator11 of the step motor.

Described next is the result of experiment that is performed forchecking the relation between the initial phase angle θ₁ and the holdingtorque when the installation angles θ₂, θ₃ of the two pairs of recesses15 c, 15 d and 15 e, 15 f are respectively varied in the step motorusing the stator 11 having the foregoing construction.

The experiment is performed using a rotary information measuring deviceas disclosed in International Patent Publication No. WO 98/30869,wherein a holding torque is determined by solving equation of motionbased on the change of angular velocity of the rotor relative to thedisplacement angle of the rotor that is directly measured.

Further, the initial phase angle θ₁ is determined by the displacementangle of the rotor that is directly measured and a counter electromotiveforce that is measured at the same time with the variation of an angularvelocity relative to the displacement angle of the rotor.

The step motor used in the measurement uses the stator 11 as explainedwith reference to FIG. 6 by which the stator of known step motor for atimepiece is replaced, by which the initial phase angle θ₁ and theholding torque are respectively measured.

The recesses 15 c, 15 d and recesses 15 e, 15 f of the stator 11 of thestep motor used in the measurement are formed by electric dischargemachining while press working is not applied thereto at the positionwhere the installation angles θ₂, θ₃ are formed before magneticannealing is applied.

Thereafter, a plurality of stators 11 are prepared wherein theinstallation angle θ₂ of one pair of recesses 15 c, 15 d is set to 15degrees while the installation angle θ₃ of the other pair of recesses 15e, 15 f is set to a range from 45 degrees to 90 degrees that is variedin increments of 15 degrees, and an initial phase angle θ₁ and a holdingtorque of the step motor incorporating therein these stators 11 aremeasured.

All the recesses 15 c to 15 f of the respective stators 11 have thewidth Wc of 400 μm, the depth D of 150 μm.

A plurality of stators 211 are prepared to compare with a step motor fortimepiece having the conventional construction wherein each width Wc ofone pair of recesses 205 a, 205 b is set to 400 μm, each depth D is setto 150 μm, an installation angle θ₁₂ is set to range from 30 to 75degrees in increments of 15 degrees, and the initial phase angle θ₁(refer to FIG. 22) and the holding torque of the step motorincorporating these stators 211 therein are measured.

According to the measurement result, in the step motor incorporatedtherein the stator 211 having the conventional construction, the initialphase angle θ₁ is conformed to the installation angle θ₁₂ within ameasuring error. Further, the maximum holding torque is 250 nNm and isfixed to this value without depending on the installation angle θ₁₂.

From this measurement result, it has been found that in the step motorhaving the construction using the conventional stator 211, the initialphase angle θ₁ is determined by the installation angle θ₁₂ and themaximum holding torque is not dependent on the installation angle θ₁₂.

Described next is a result of experiment for checking the relationbetween the dimensions of a pair of recesses and the maximum holdingtorque.

With the experiment, the stator 211 with a pair of recesses 205 a, 205 bas shown in FIG. 24 is used, and a plurality of stators 211 are preparedwherein each width Wc of the respective recesses 205 a, 205 b is fixedto 400 μm, and the depths thereof are differentiated from one another,namely, they are set to 50, 100, 150 or 200 μm.

Further, a plurality of stators 211 are also prepared wherein each depthD of the respective recesses 205 a, 205 b is fixed to 150 μm and thewidths Wc are differentiated from are another, namely, they are set to100, 200, 300 or 400 μm.

Then, each holding torque of the step motor incorporated therein thesestators 211 are subsequently measured.

The measurement result is shown in FIGS. 9 and 10.

It has been found that the maximum holding torque increases sharply aseach depth D of respective recesses increases as shown in FIG. 9, and itincreases gently when the depth becomes in the order of 100 μm ordeeper. It has been also found that the maximum holding torque increaseslinearly as each width Wc of respective recesses increases as shown inFIG. 10, and it is proportional to the width Wc of the recesses.

From this result of the experiment, it has been found that the maximumholding torque is not proportional to the sum of areas of the recessesas disclosed in the literatures and the like. That is, the stator 211used in this experiment has a pair of recesses 205 a, 205 b, and the sumof the areas of the recesses 205 a, 205 b becomes double the product ofthe width Wc and the depth D of one of the recesses.

Accordingly, since each width Wc of the recesses is fixed to 400 μm inthe stator 211 used in the experiment as shown in FIG. 9, if the maximumholding torque is proportional to each sum of areas of the recesses, itshould be proportional to the depth D of the recesses.

Meanwhile, in the measurement result shown in FIG. 9, the relationbetween the maximum holding torque and each depth D of the recesses isnot a linear line representing the proportional relation. Thismeasurement result represents that the maximum holding torque is notproportional to the sum of the areas of the recesses but can be set bychanging each depth D of each recess in the region where each recess hasa depth of 100 μm or less, and the maximum holding torque becomessubstantially constant if the depth D exceeds 100 μm.

When the maximum holding torque is adjusted to be small, if theadjustment is performed by changing the depth D of the recess, thechange of the maximum holding torque is large relative to the change ofthe depth D of the recess at the region where the depth D of therecesses is in the order of 100 μm or less, particularly at the regionwhere the depth D is 50 μm or less, as evident from FIG. 9. Accordingly,considering the precision with which to process the stator, it ispreferable not to adjust the maximum holding torque at the region wherethe depth D is 50 μm or less because it is difficult to set the maximumholding torque stably.

On the other hand, as shown in FIG. 10 showing the measurement result inthe case of the depth D of the recess being 150 μm, the maximum holdingtorque is substantially proportional to the width Wc of the recesses ifthe depth D of the recess is about 150 μm or more, from which it hasbeen found that the change of the width Wc is suitable for adjusting themaximum holding torque.

In such a manner, it has been found that although the maximum holdingtorque does not depend on the installation angle θ₁₂ as explained withreference to FIG. 24, it is determined by each of width Wc and depth Dof the pair of recesses 205 a, 205 b.

Meanwhile, the measurement result of the step motor incorporating thestator 11 therein as explained with reference to FIG. 6 shows that it isdifferent from the measurement result of the step motor incorporatingtherein the conventional stator 211 as shown in FIG. 8.

That is, the initial phase angle θ₁ does neither conform to theinstallation angle θ₂ of one pair of recesses 15 c, 15 d nor to theinstallation angle θ₃ of another pair of recesses 15 e, 15 f.

Further, the maximum holding torque does not become constant at the sumvalue 500 nNm of double the maximum holding torque 250 nNm obtained fromthe pairs of the recesses 15 c, 15 d and the recesses 15 e, 15 f each.The maximum holding torque decreased from 430 nMm to 130 nNm within therange of the measurement at this time, accompanied with the increase ofthe setting angle θ₃ of the paired recesses 15 e, 15 f.

Thus, in comparison of the measurement results of the step motor inwhich the conventional stator 211 and the stator 11 explained withreference to FIG. 6 were assembled, a totally different phenomenon wasobserved in the set initial phase angle and the holding torque, when thestep motor using the stator 11 having the recesses 15 c to 15 e formedas a plurality of holding torque setting means was compared with thestep motor using the conventional stator 211.

When these measurement results were examined still further, in the stepmotor using the stator 11 provided with two pairs of the recesses 15 c,15 d and the recesses 15 e, 15 f being the plurality of holding torquesetting means, the initial phase angle θ₁ and the maximum holding torquewere found to become composition of vectors of the initial phase angleθ₁ and the maximum holding torque obtained when each of the holdingtorque setting means, namely, the pair of the recesses 15 c, 15 d andthe pair of the recesses 15 e, 15 f is individually formed on the innerperiphery of the rotor hole 2.

Further, since the step motor for a timepiece in the second embodimentis a two-pole step motor of which displacement angle by one step is not360 degrees, but 180 degrees, the electrical angle of the step motorbecomes double the actual angle.

Accordingly, each of the vectors corresponding to the individual holdingtorque setting means is given by doubling the initial phase angle θ₁ andthe maximum holding torque obtained when each holding torque settingmeans is individually provided on the inner periphery of the rotor hole2.

Further, from the measurement results of the step motor using theconventional stator 211, the installation angles θ₂ and θ₃ of the twopairs of the recesses 15 c, 15 d and the recesses 15 e, 15 f serving asthe holding torque setting means may be used instead of the initialphase angle θ₁.

In this manner, with the step motor using the stator 11 having aplurality of holding torque setting means, the vector composition ofeach vectors obtained as above allows to set the initial phase angle θ₁and the maximum holding torque. And, the initial phase angle θ₁ and themaximum holding torque set by the vector composition well coincided withthe measurement results of the experiment made at this time.

That is, when the two holding torque setting means, the pair of therecesses 15 c, 15 d and the pair of the recesses 15 e, 15 f each areindividually formed, even when the holding torque setting means eachform the maximum holding torque of 250 nNm, when the installation angleθ₂ of the pair of the recesses 15 c, 15 d is 15 degrees, theinstallation angle θ₃ of the pair of the recesses 15 e, 15 f is 90degrees, and the difference between the installation angles θ₂ and θ₃ is75 degrees, the maximum holding torque becomes about 130 nNm and theinitial phase angle θ₁ becomes about 53 degrees from the measurementresults shown in FIG. 8.

As mentioned above, according to the step motor using the stator 11having two pairs of recesses 15 c, 15 d and recesses 15 e, 15 f of thesecond embodiment of the invention, it is possible to set the holdingtorque of a wider range and the initial phase angle θ₁ by adjusting themaximum holding torque which can be set by the holding torque settingmeans independently of each other based on the width Wc and depth D ofrespective pairs of recesses 15 c to 15 f and the installation anglesθ₂, θ₃ forming the positions where respective recesses 15 c to 15 f areformed.

As a result, when respective installation angles θ₂, θ₃ of the two pairsof recesses 15 c, 15 d and recesses 15 e, 15 f are adjusted and thedifference between these installation angles is made large, in otherword, when the difference between phase angles of each holding torque ismade large, each holding torque established when forming the two pairsof recesses 15 c, 15 d and recesses 15 e, 15 f solely are large, but theentire holding torque of the step motor becomes composition of vectorsof each holding torque, and hence the holding torque obtained finallycan be reduced to the extremely small value.

Described next is a result of comparison obtained by an experiment forconfirming a power consumption of the step motor for a timepiece inwhich a stator having two pairs of recesses is incorporated and that ofthe step motor for a timepiece in which a stator having one pair ofrecesses is incorporated.

Both the stator having two pairs of recesses and the stator having onepair of recesses are formed respectively so as to have the initial phaseangle of about 55 degrees and the maximum value of the holding torque isabout 75 nNm. A material of these stator is permalloy having a thicknessof 500 μm, the diameter of the rotor hole is 1700 μm, and a samariumcobalt magnet having an outer diameter of 1000 μm and a thickness of 400μm is used as the rotor magnet.

First, as the stator 11 having the two pairs of recesses 15 c, 15 d andrecesses 15 e, 15 f, each width Wc of the respective recesses 15 c to 15f (FIG. 7) is set to 400 μm, each depth D thereof is set to 150 μm whilethe installation angle θ₂ of one pair of recesses 15 c, 15 d is set to15 degrees and the installation angle θ₃ of the other pair of recesses15 e, 15 f is set to 96 degrees, thereby realizing a step motor havingthe initial phase angle θ₁ of 55 degrees and the maximum holding torqueof 75 nNm.

On the other hand, it has been found that the stator 211 having theconventional one pair of recesses 205 a, 205 b need be set such thateach width Wc of respective recesses 205 a, 205 b is in the order of 120μm and each depth D thereof is in the order of 150 μm, and theinstallation angle θ₁₂ is set to be 55 degrees.

As a measuring result of power consumption of the step motorsincorporating therein respectively two kinds of stators 11 and 211 whilethey are actually driven, it has been found that an ordinary step motorfor a timepiece having the maximum holding torque of about 250 nNm showsa power consumption of about 800 to 900 nJ for one step while the stepmotor incorporating therein the stators 11 and 211 shows a powerconsumption of about 400 nJ, resulting in achieving a lower powerconsumption.

Further, there is no difference in characteristics between the stepmotors incorporating therein the stators 11 and 211.

As a result of the foregoing experiments, if the same characteristicsare obtained by the step motor incorporating therein a stator having twopairs of recesses and the step motor incorporating therein a statorhaving one pair of recesses, with the step motor incorporating thereinthe stator 11 having two pairs of recesses 15 c, 15 d and recesses 15 e,15 f, each width Wc of two pairs of recesses 15 c to 15 f is set to 400μm and each depth D is set to 150 μm.

On the other hand, with the conventional step motor incorporatingtherein the stator 211 having one pair of recesses 205 a, 205 b, eachwidth Wc of the recesses 205 a, 205 b is set to 120 μm and the depth Dis set to 150 μm so that it can obtain the same characteristics as thestep motor incorporating therein the stator having two pairs ofrecesses.

Accordingly, each width Wc of the respective recesses 205 a, 205 bshould be considerably small compared with the stator 11 having twopairs of recesses.

For this, there occurs a problem in the stator 211 having one pair ofrecesses in view of machining and productivity.

That is, since two kinds of stators 11 and 211 used in the experimentare formed by electric discharge machining capable of processing thereofto form respective recesses with high precision so as to obtain theresult of experiment with high precision, there does not occur a problemin machining the stators to form the recess to have a width of 120 μm.

However, when the stators are manufactured actually in a factory, it isnecessary that they can be manufactured by press working considering aproductivity. Meanwhile, it is very difficult to apply press working toa band material having a thickness of 500 μm and made of permalloy withprecision to form each recess having the width of 120 μm and the depthof 150 μm on the band material. Even if such a machining is possible, amold to be used is very short in service life, causing an extremelyinferior productivity.

Even if the stators are manufactured by electric discharge machininghaving high precision, it takes time for machining, resulting in veryexpensive stators.

On the other hand, according to the stator 11 having two pairs ofrecesses 15 c, 15 d and recesses 15 e, 15 f as explained with referenceto FIG. 6, it is possible to manufacture the step motor having theinitial phase angle θ₁ of 55 degrees and the maximum holding torque of50 nNm by adjusting the installation angle θ₂ of one pair of recesses 15c, 15 d to 13 degrees and installation angle θ₃ of the other pair ofrecesses 15 e, 15 f to 97 degrees while each width Wc of two pairs ofrecesses 15 c to 15 f is set to 400 μm and each depth D thereof is setto 150 μm.

Although the stator 11 according to the second embodiment of theinvention is exemplified to have two pairs of recesses 15 c, 15 d andrecesses 15 e, 15 f provided on the inner periphery of the rotor hole 2as the holding torque setting means, the recesses serving as the holdingtorque setting means may be provided on the inner periphery of the rotorhole 2 by three or more pairs.

Particularly, in the case where an axle hole of gears or holes of fixingpins are formed around the rotor hole of the stator, such axle hole ofgears or holes of fixing pins are prone to interfere with the recessesserving as the holding torque setting means, resulting in arising apossibility that the recesses can not be provided at the intendedinstallation angles so as to obtain the initial phase angle and theholding torque to be set.

In such a case, each holding torque established in the case where thepair of recesses are formed at the positions to interfere with the axlehole of gears or holes of fixing pins are subjected to a vectordecomposition as described above, and three or more pairs of recessesare disposed separately at the positions where they do not interferewith the axle hole of gears or the holes for fixing pins so that thepositions conform to the decomposed vectors, thereby obtaining anintended initial phase angle and the holding torque.

Although typically explained in the second embodiment with reference toFIGS. 5 and 6 is a case where two pairs of recesses 15 c to 15 f havingsubstantially the same dimensions and rectangular shapes are provided astwo holding torque setting means on the inner periphery of the rotorhole 2, the two or more pairs of recesses may have any shape if theyhave openings relative to the rotor hole 2, and if they have the sameshapes and dimensions as the pairing recesses, they may be changed inshapes and dimensions every pair of recesses.

Third Embodiment: FIGS. 11 and 12

A two-pole step motor for a timepiece according to a third embodiment ofthe invention is now described with reference to FIGS. 11 and 12.

FIG. 11 is a plan view of the two-pole step motor for a timepiece of thethird embodiment like FIG. 1, and FIG. 12 is a plan view showing theconstruction of a stator of the two-pole step motor for a timepiece inFIG. 11.

The two-pole step motor for a timepiece of the third embodiment is thesame as the two-pole step motor for a timepiece of the first embodimentof the invention as explained with reference FIG. 1 to 4, except theconstruction of a stator, and hence the explanation of the samecomponents is omitted.

The two-pole step motor for a timepiece according to the thirdembodiment of the invention as shown in FIG. 11 has connections 31 c, 31d of a stator 31 forming at least one of a plurality of holding torquesetting means, which is the same as the step motor shown in FIG. 1having connections 1 c, 1 d of the stator 1 as explained with referenceto FIG. 1, but the third embodiment is different from the firstembodiment in that one pair of recesses 35 a, 35 b formed ofsubstantially rectangular notches serving as holding torque settingmeans are formed at positions different from the positions where theconnections 31 c, 31 d are provided on the inner periphery of a rotorhole 2 of the stator 31.

The recesses 35 a, 35 b are formed symmetrically with respect to thecenter of the rotor hole 2.

Although the installation angle of the connections 31 c, 31 d shown inFIG. 11 is 0 degrees, it is not always 0 degrees.

The method of manufacturing the stator 31 is the same as that of thestator 1 as explained in FIG. 2, and it is formed of a band materialhaving a thickness of 500 μm and made of permalloy which is ahigh-permeability material, and press working is applied to the bandmaterial to form a pilot hole, a prepared hole for the rotor hole 2 andfixing holes 6, 6, and the band material is punched to form an outerconfiguration of the stator 31 while leaving a connecting portion (notshown) for connecting to the band material at a part thereof.

Subsequently, a slit is formed on the portion where the connections 31c, 31 d are formed, and a wire rod made of a low-permeability materialor nonmagnetic material is inserted into the slit, then the stator part31 a and stator part 31 b are bonded to each other by laser weldingthrough the intermediary of the wire rod.

Thereafter, the rotor hole 2 and the parts of the recesses 35 a, 35 bare punched by press working, and finally the portion to be coupled tothe band material is punched, thereby completing the external shapeworking of the stator 31. Thereafter, the magnetic annealing is appliedto the band material to make up the stator 31 of the step motor.

With the stator 31 having the forgoing construction, the connections 31c, 31 d are formed such that a high permeability material are notched bypress working and a low-permeability material or nonmagnetic material isbonded onto the notch by welding, hence these portions function like theholding torque setting means like a pair of recesses 35 a, 35 b to holda rotor 3 at given positions.

As a result of confirming the step motor incorporating therein thestator 31, it has been found that the initial phase angle θ₁ of theholding torque established by the connections 31 c, 3 d conformssubstantially to an installation angle of the connections 31 c, 31 dlike the pair of recesses 35 a, 35 b, and the magnitude of the thusestablished holding torque conforms substantially to a magnitude of thethus established holding torque conforms substantially to a holdingtorque established by the recesses of the same dimensions.

From the foregoing, even in the construction of the stator 31 like thestep motor as explained with reference to FIG. 1 to 4, a holding torqueset by composition of vectors of holding torque by providing theconnections 31 c, 31 d solely on the inner periphery of the rotor hole 2and vectors of respective holding torque established by providing thepair or recesses 35 a, 35 b solely on the inner periphery of the rotorhole 2 might become finally a setting holding torque of the step motor.

An experiment to confirm this is explained hereinafter.

As the stator 31 to be used in this experiment is a stator having a pairof connections 31 c, 31 d and a pair of recesses 35 a, 35 b respectivelyformed on the inner periphery of the rotor hole 2 shown in FIG. 12.

An installation angle θ₄ of the pair of recesses 35 a, 35 b shown inFIG. 12 and each width Wc of the recesses 35 a, 35 b are adjusted in aposition where the initial phase angle θ₁ (FIG. 11) becomes about 55degrees and the maximum holding torque becomes about 75 nNm.

As a material of the stator 31, a permalloy having a thickness of 500 μmis used and a diameter of the rotor hole 2 is set to 1700 μm. Further,as the rotor magnet 3 a, a samarium cobalt magnet having an outerdiameter of 1000 μm and a thickness of 400 μm is used.

As a measurement result of the initial phase angle θ₁ and holding torqueof the stator 31 formed as such and incorporated in the step motor for atimepiece, each width Wc of the pair of recesses 35 a, 35 b is set to270 μm and each depth D thereof is set to 150 μm (refer to FIG. 7), andthe installation angle θ₄ of the pair of recesses 35 a, 35 b is set to78 degrees, so that a step motor having the initial phase angle θ₁ ofabout 55 degrees and the maximum holding

The result of experiment shows that the holding torque of the step motoris set by composition of vector of a holding torque established by theconnections 31 c, 31 d and the vector of a holding torque established bythe pair of recesses 35 a, 35 b provided on the inner periphery of therotor hole 2.

If the stator 31 of the third embodiment of the invention is used as setforth above, since each width Wc of the recesses 35 a, 35 b is set to270 μm to realize a step motor having the initial phase angle θ₁ ofabout 55 degrees and the maximum holding torque of about 75 nNm, it canenlarge each width of the recesses remarkably compared with each widthWc of 120 μm in the recesses of the conventional stator 211.Accordingly, the stator 31 can be processed with ease.

Further, if the stator 31 is used, there is an advantage that each widthWc of the pair of recesses 35 a, 35 b can be widened by widening eachwidth Wb of the connections 31 c, 31 d.

For example, although the foregoing stator 31 used the connections 31 c,31 d having a width Wb of 200 μm if the width Wb is set to wider value,i.e., 400 μm, sizes of the recesses 35 a, 35 b necessary for setting theinitial phase angle θ₁ to about 55 degrees and the maximum holdingtorque to about 75 nNm are preferably that each width Wc of the recessesis set to 450 μm, each depth D is set to 150 μm, and the installationangle θ₄ is set to 83 degrees.

That is, when setting the same initial phase angle θ₁ and holdingtorque, if each width Wb of connections is widened from 200 μm to 400μm, each width Wc of the recesses 35 a, 35 b can be widened from 270 μmto 450 μm.

If each width Wb of the connections 31 c, 31 d of the stator 31 is setto 400 μm, and the installation angle θ₄ of the pair of recesses 35 a,35 b is set to 85 degrees, and each width Wc of the recesses is set to430 μm, the initial phase angle θ₁ can be set to 55 degrees and themaximum holding torque can be set to 50 nNm like the foregoing secondembodiment.

Described next is a measurement result of a power consumption for onestep when the step motor for a timepiece incorporated therein the stator31 is actually driven.

The measurement result of the power consumption of the step motorincorporated therein the stator 31 for one step was about 350 nJ. Thisvalue is considerably small power consumption compared with a powerconsumption of 800 to 900 nJ for one step of an ordinary step motor fora timepiece having the maximum holding torque of about 250 nNm.

Achievement of lowering power consumption of about 350 nJ for one stepof the step motor of the third embodiment is aimed by 50 nJ comparedwith the power consumption of about 400 nJ of the step motor for atimepiece using the stator 11 of the second embodiment of the inventionin which the same holding torque is set.

This is considered to be caused by the function of the nonmagneticconnections 31 c, 31 d in the construction of the stator 31.

As set forth above, if the stator 31 according to the third embodimentof the invention is used, it is possible to set an extremely smallholding torque suitable for achieving of lowering power consumptionwithout impairing a productivity like the stator 11 of the secondembodiment of the invention.

Although explained with reference to FIGS. 11 and 12 in the thirdembodiment is the case where the pair of recesses 35 a, 35 b are formedon the inner periphery of the rotor hole 2 other than the connections 31c, 31 d serving as the holding torque setting means, the number ofrecess to be formed on the inner periphery of the rotor hole 2 may betwo pairs or more.

The shapes of the recesses 35 a, 35 b formed on the inner periphery ofthe rotor hole 2 are not limited to substantially rectangular shapes ofthe pair of recesses as shown in FIG. 12, but they may have any shape ifthey have openings relative to the rotor hole 2 suppose they have thesame shapes and dimensions as the pair of recesses.

Fourth Embodiment: FIG. 13

A two-pole step motor for a timepiece according to a fourth embodimentof the invention is described with reference to FIG. 13.

FIG. 13 is a plan view like FIG. 6 showing the construction of a statorof a two-pole step motor for a timepiece according to the fourthembodiment of the invention.

The two-pole step motor for a timepiece according to the fourthembodiment of the invention is the same as that of the second embodimentexplained with reference to FIGS. 5 and 6 except the construction of astator, and hence the same components are omitted to explain.

Although a stator 41 shown in FIG. 13 has a plurality of holding torquesetting means for holding a rotor 3 at given positions in the rotarydirection, the plurality of holding torque setting means include holdingtorque setting means having an asymmetrical shape with respect to thecenter of the rotor hole 2. That is, in the stator 41, one pair ofrecesses 45 a, 45 b and another pair of recesses 45 c, 45 d are formedon an inner periphery of the rotor hole 2 serving as holding torquesetting means for establishing a holding torque and an initial phaseangle of the rotor 3.

The recesses 45 a, 45 b are disposed symmetrically relative to thecenter of the rotor hole 2. Likewise, the recesses 45 c, 45 d aredisposed symmetrically with respect to the center of the rotor hole 2.

Although the recesses 45 a, 45 b have the same shape as is evident fromFIG. 13, the recesses 45 c, 45 d have not the same shape, wherein awidth Wc of the recess 45 c is larger than that of the recess 45 d.

An angle formed by a straight line 27 passing through the center of therotor hole 2 and orthogonal to an excitation direction line (a magneticfiled directed line 12 in FIG. 5) and a straight line 64 passing throughthe centers of the pair of recesses 45 a, 45 b is defined as aninstallation angle θ₅ of the recesses 45 a, 45 b. Likewise, an angleformed by a straight line 62 passing through the centers of the recesses45 c, 45 d and the straight line 27 is defined as a installation angleθ₆ of the recesses 45 c, 45 d. The installation angles θ₅, θ₆ have apositive value in counterclockwise direction.

Meanwhile, in the stator 11 shown in FIG. 6, the holding torque settingmeans comprises two pairs of recesses 15 c to 15 f, and the holdingtorque and initial phase angle formed by the paired recesses arerepresented by vectors every paired recesses, and the maximum holdingtorque that is set as a consequence becomes the sum of these vectors.

Since all the respective recesses 15 c to 15 f have the same shapes andsame sizes, two vectors of each pair of recesses have the samemagnitude, while the phase angles are determined based on installationangles θ₂, θ₃ of each pair of recesses.

On the other hand, in the stator 41 of the fourth embodiment of theinvention, one pair of recesses 45 a, 45 b and another pair of recesses45 c, 45 d correspond to respective vectors each having a holdingfunction, and composition of holding functions of two pairs of recesses45 a to 45 d becomes composition of these vectors.

Here, stators, namely, one having a pair of recesses, and others havinga pair of recesses which are not the same in size have been manufacturedand an experiment was performed for checking each holding torque of thetesters.

The stator used in this experiment having the same dimensions of therecesses has each width Wc of the recesses set to 400 μm in accordancewith the experiment that was performed when obtaining the experimentaldata as shown in FIG. 8. The stators having different dimensions have acombination of the width Wc of recesses set to 450 μm and 350 μm andanother combination of the width Wc of the recesses set to 500 μm and300 μm so that the sum of the width in each combination becomes 800 μm.

Each depth D of the recesses is set to 150 μm. An experiment of the stepmotor incorporating these stators built therein shows that a holdingtorque and an initial phase angle of the rotor conform each other withina measurement error.

It has been confirmed in the experiment that even if the shape of therecesses (same in the case of protuberances) is asymmetrical, if the sumof each width Wc of two recesses is fixed, the holding torque andinitial phase angle of the rotor can be kept constant.

Accordingly, although there is a difference in size between respectiveareas of the pair of recesses 45 c, 45 d in the case of the stator 41shown in FIG. 13, when the areas of the recesses are changed under thecondition that each depth D of the recess is fixed and each width Wcthereof is changed, if the sum of the areas of a pair of recesses isfixed, a vector is basically the same, and asymmetrical recesses do notaffect the vector.

Accordingly, by adjusting the sum of areas of a plurality of pairedrecesses, the vectors can be adjusted, while when the vectors, i.e., thesum of the areas of the recesses, is fixed, the areas of a plurality ofpaired recesses can be allotted to adjust thereof.

In the case of the stator 41 shown in FIG. 13, one pair of recesses 45c, 45 d are differentiated in a size to differentiate in an area, butnot only one pair of recesses 45 c, 45 d but also the other pair ofrecesses 45 a, 45 b can be differentiated in a size to differentiate inan area.

In such a manner, the allocation of areas of the pair of recesses 45 c,45 d and the selection of the installation angle θ₅ of the pair ofrecesses 45 a, 45 b and installation angle θ₆ of the pair of recesses 45c, 45 d increase the degree of freedom for disposing the recessescompared with the conventional case where the paired recesses have thesame shape. Accordingly, the holding torque or initial phase angle canbe set more conveniently.

Fifth Embodiment: FIG. 14

A two-pole step motor for a timepiece according to a fifth embodiment ofthe invention is now described with reference to FIG. 14.

FIG. 14 is a plan view showing the construction of a stator of thetwo-pole step motor for a timepiece according to the fifth embodiment ofthe invention like FIG. 12, and the components corresponding to those inFIG. 12 are depicted by the same reference numerals.

In a stator 51 used in the two-pole step motor for a timepiece accordingto the fifth embodiment, the shape of the recess 55 a side is madelarger than the recess 55 b side among the-pair of recesses 55 a and 55b which are formed on the inner periphery on the rotor hole 2symmetrically with respect to the center of the rotor hole 2 comparedwith the stator 31 as explained with reference to FIG. 12.

The first and second stator parts 51 a, 51 b made of respectivelyhigh-permeability material are integrally connected to each otherthrough the intermediary of connections 31 c, 31 d made oflow-permeability material or nonmagnetic material by welding, whereinthe connections 31 c, 31 d operate in the same manner as the pairs ofrecesses 55 a, 55 b to function as the holding torque setting means.

Accordingly, it is considered that the stator 51 has substantially twopairs of recesses, so that the maximum holding torque and the initialphase angle produced in the rotor 3 become a sum of a vectorcorresponding to a holding torque established by the pair of recesses 55a, 55 b and a vector corresponding to a holding torque established bythe connections 31 c, 31 d.

In the case the stator 51 is adopted, the restriction for making therecesses 55 a, 55 b same in size is removed so that the recesses 55 a,55 b can be disposed with ease. Further, a holding torque and an initialphase angle of the rotor 3 can be set extremely freely by merelyselecting an installation angle θ₇ formed by a central line 63 passingthrough the recesses 55 a, 55 b relative to a straight line 27 passingthrough the centers of the connections 31 c, 31 d.

With the stator 51 shown in FIG. 14, suppose an angle formed by astraight line passing through the centers of the connections 31 c, 31 d(conforming to the straight line 27 in this case) and the straight line27 passing through the center of the rotor hole 2 and orthogonal to astraight line in the excitation direction of the stator 51 is aninstallation angle of the connections 31 c, 31 d, the installation anglebecomes 0 degree. However, the installation angle may be other than 0degree and the connections 31 c, 31 d may be formed in a tiltedposition.

Further, a width Wb of the connection 31 c may be differentiated from awidth Wb of the connection 31 d. In such a case, the characteristics ofthe step motor can be more freely set.

An experiment is performed for checking a holding torque in the casewhere one of paired recesses is formed, namely, only one recess isformed on the inner periphery of the hole as an extreme case of theholding torque setting means having different shapes from those of theforegoing paired recesses (a case of protuberance, described later, isthe same).

According to the result of the experiment, even if there is providedonly one recess, a holding torque is set in the same manner as a holdingtorque is set by a pair of recesses. It has been found that one vectorrepresenting the maximum holding torque and initial phase anglecorresponds to one recess, and if a plurality of recesses are formed,vectors corresponding thereto become composition of respective vectorsin whole.

It has been found from this experiment that as a base of holding torquesetting means, it does not necessarily follow that the holding torquesetting means is provided on the paired recesses, but even if a solerecess which is not paired may be disposed on the inner periphery of therotor hole at a desired position, desired characteristics can beobtained.

Meanwhile, if two holding torque setting means are providedsymmetrically with respect to the center of the rotor hole,theoretically the rotor receives only a holding torque from the stator,and hence there does not produce a side pressure at the bearing side.

However, a side pressure is produced in the rotor as two holding torquesetting means are displaced to a position where they become asymmetricalwith respect to the center of the hole. The side pressure is mostintense when a sole recess serving as holding torque setting means isformed, and friction on the bearing part increases by the side pressure.Accordingly, a current for driving the motor is prone to slightlyincrease to oppose the production of the friction. However, there aremany cases where several holding torque setting means are combined withone another and disposed in a practical use. In such a case, the sidepressure will be offset, and hence a current for driving the rotor doesnot increase much finally.

From the above experiment, it has been found that one vectorrepresenting the holding torque and initial phase angle is presentrelative to one recess, and the vector when a plurality of sole recessesare formed without paired becomes composition of these vectors.

Accordingly, recesses formed on the inner periphery of the hole as theholding torque setting means are not always to be paired. That is, asole recess which is not paired forms a basic unit of the holding torquesetting means and such a recess may be disposed on the inner peripheryof the hole by necessary number.

Sixth Embodiment: FIG. 15

A two-pole step motor for a timepiece according to a sixth embodiment ofthe invention is described now with reference to FIG. 15.

FIG. 15 is a plan view showing the construction of a stator of thetwo-pole step motor for a timepiece according to the sixth embodiment ofthe invention like FIG. 13, and the components corresponding to FIG. 13are depicted by the same reference numerals.

With a stator 61 used by the two-pole step motor for a timepieceaccording to the sixth embodiment of the invention, a pair of recesses45 a, 45 b are formed on the inner periphery of a rotor hole 2 at aposition where a straight line 64 passing through the centers of therecesses 45 a, 45 b forms an installation angle θ₅ relative to astraight line 27, and a sole recess 45 c which is not paired is formedat a position where a straight line 65 passing through the center of therecess 45 c forms an installation angle θ₆ relative to the straight line27.

The holding torque set by the stator 61 becomes composition of a vectorcorresponding to a holding torque established by a pair of recesses 45a, 45 b and a vector corresponding to a holding torque established inthe sole recess 45 c.

As mentioned above, the holding torque setting means is not limited to acombination of paired recesses, and even if the sole recess is disposedat various positions, desired characteristics can be obtained.

Seventh Embodiment: FIG. 16

A two-pole step motor for a timepiece according to a seventh embodimentof the invention is described now with reference to FIG. 16.

FIG. 16 is a plan view showing the construction of a stator of thetwo-pole step motor for a timepiece according to the seventh embodimentof the invention like FIG. 15, and the components corresponding to FIG.15 are depicted by the same reference numerals.

A stator 71 used in the two-pole step motor for a timepiece according tothe seventh embodiment of the invention has recesses 45 c, 45 e at twopositions on the inner periphery of the rotor hole 2 and these recesses45 c, 45 e are neither paired nor symmetrically provided with respect tothe center of the rotor hole 2 but each of them is a sole recess. Astraight line 65 passing through the center of the recess 45 c forms aninstallation angle θ₆ relative to a straight line 27 while a straightline 64 passing through the center of the recess 45 e forms aninstallation angle θ₅ relative to the straight line 27.

With the stator 71, a state of holding the rotor 3 is determined bycomposition of vectors of respective recesses 45 c, 45 e for holding therotor 3.

Eighth Embodiment: FIG. 17

A two-pole step motor for a timepiece according to an eighth embodimentof the invention is described now with reference to FIG. 17.

FIG. 17 is a plan view showing the construction of a stator of thetwo-pole step motor for a timepiece according to the eighth embodimentof the invention like FIG. 14, and the components corresponding to FIG.14 are depicted by the same reference numerals.

A stator 81 used in the two-pole step motor for a timepiece according tothe eighth embodiment of the invention comprises first and second statorparts 81 a, 81 b made of high-permeability material respectively whichintegrally connected to each other by welding through the intermediaryof a pair of connections 31 c, 31 d made of low-permeability material ornonmagnetic material each having a width Wb.

A sole recess 55 a is formed on the inner periphery of a rotor hole 2and a straight line 63 passing through the center of the recess 55 aforms an installation angle θ₇ relative to a straight line 27.

Also in the stator 81, likewise the stator 51 as explained withreference to FIG. 14, the pair of connections 31 c, 31 d operate likepaired recesses and have vectors for holding rotor. Since the solerecess 55 a also has a vector, a holding torque and an initial phaseangle of the rotor 3 are determined by composition of these vectors.

Also in the stator 81 shown in FIG. 17, a straight line passing throughthe centers of the connections 31 c, 31 d is superimposed on a straightline 27, so that the installation angle of the connections 31 c, 31 dbecomes 0 degree in the same manner as the stator 51 shown in FIG. 14,but the installation angle may be set to an angle other than 0 degreeand the connections 31 c, 31 d may be tilted in the same manner as thecase of the stator 51 as explained above.

Further, the width Wb of the connections 31 c, 31 d may bedifferentiated from each other. Still further, the centers of theconnections 31 c, 31 d may not be positioned on the same straight line.

Ninth Embodiment: FIG. 18

A two-pole step motor for a timepiece according to a ninth embodiment ofthe invention is described now with reference to FIG. 18.

FIG. 18 is a plan view showing the construction of a stator of thetwo-pole step motor for a timepiece according to the ninth embodiment ofthe invention like FIG. 15, and the components corresponding to FIG. 15are depicted by the same reference numerals.

A stator 91 used in the two-pole step motor for a timepiece according tothe ninth embodiment of the invention comprises different types ofholding torque setting means, namely, gap type and notched type holdingtorque setting means.

That is, in the stator 91, stepped parts 94 c, 94 d each having a gapamount G serving as the gap type holding torque setting means and a pairof recesses 45 a, 45 b serving as notched type holding torque settingmeans are respectively provided in a rotor hole 92.

With the step motor using the stator 91, a holding torque and initialphase angle of the rotor are determined by composition of a vectorcorresponding to holding torque established by the stepped parts 94 c,94 d and a vector corresponding to holding torque established by thepair of recesses 45 a, 45 b.

Meanwhile, in a simple gap type stator as shown in the conventionalstator 201 shown in FIG. 23, suppose a diameter of the hole is about1700 μm on average and a gap amount G is 40 μm, the maximum holdingtorque of the rotor becomes about 300 nNm and an initial phase anglebecomes about 45 degrees.

However, as a result of experiment made by the inventors, an optimuminitial phase angle of the rotor (refer to θ₁ in FIG. 22) for enhancingan efficiency of the step motor to lower a power consumption is largerthan 45 degrees, more particularly, it ranges from 50 to 60 degrees, butit may be up to 70 degrees which does not cause any problem in practicaluse.

Whereupon if the stepped parts 204 a, 204 b are provided at the positionwhere they are, e.g., rotated 20 degrees counterclockwise from aposition on the straight line 27 orthogonal to its longitudinaldirection of a stator 201 as viewed in FIG. 23, the initial phase anglecan be approached to the foregoing ideal angles of 50 to 60 degrees.

However, if the maximum holding torque is intended to be reduced toachieve further lowering of power consumption, the gap amount G need beextremely small value. Accordingly, working for that becomes difficultto be performed.

Meanwhile, if the stator 91 as explained with reference to FIG. 18 isused, it can reduce the maximum holding torque and obtain an appropriateinitial phase angle without reducing the gap amount G and the dimensionsof the recesses 45 a, 45 b to an extremely small value by appropriatelyselecting the installation angle θ₅ of the recesses 45 a, 45 b becausethe stator 91 has the gap type holding torque setting means establishedby stepped parts 94 c, 94 d and the notched type holding torque settingmeans established by a pair of recesses 45 a, 45 b.

On the other hand, with the stator 91 shown in FIG. 18, since a straightline 70 passing through the stepped parts 94 c, 94 d is superimposed ona straight line 27 orthogonal to an excitation direction of the stator91, the installation angles of the stepped parts 94 c, 94 d are 0 degreewhile the pair of recesses 45 a, 45 b form an installation angle θ₅between a straight line 64 passing through the centers thereof relativeto the straight line 27, but the stepped parts 94 c, 94 d may beprovided at an installation angle other than 0 degree in a positionwhere the straight line 70 passing through the stepped parts 94 c, 94 dis tilted relative to the straight line 27.

According to a result of experiment performed by the inventors, it isconfirmed that with the stator 91 having a gap amount G of the steppedparts being 40 μm, each width Wc of the recesses being 400 μm, eachdepth D being 150 μm, the maximum holding torque is 75 nNm and theinitial phase angle is 55 degrees where an installation angle of thestraight line 70 passing through the stepped parts 94 c, 94 d relativeto the straight line 27 is 75 degree and an installation angle θ₅ of therecesses 45 a, 45 b is −10 degree.

Alternatively, when the installation angle of the stepped parts 94 c, 94d is set to 63 degrees and the installation angle of the recesses 45 a,45 b is set to −25 degrees, it is confirmed that the maximum holdingtorque becomes 50 nNm and the initial phase angle becomes 55 degrees.

Tenth Embodiment: FIG. 19

A two-pole step motor for a timepiece according to a tenth embodiment ofthe invention is described now with reference to FIG. 19.

FIG. 19 is a plan view showing the construction of a stator of thetwo-pole step motor for a timepiece according to the tenth embodiment ofthe invention like FIG. 17, and the components corresponding to FIG. 17are depicted by the same reference numerals.

A stator 101 used in the two-pole step motor for a timepiece accordingto the tenth embodiment of the invention has a construction comprisingfirst and second stator parts 101 a, 101 b made of a high-permeabilitymaterial respectively and integrated with each other by welding throughthe intermediary of connections 31 c, 31 d made of a low-permeabilitymaterial or nonmagnetic material each having a width Wb.

A gap type rotor hole 102 is defined in the stator 101 and it hasstepped parts 94 e, 94 f.

A straight line passing through the centers of the connections 31 c, 31d is superimposed on a straight line 27 orthogonal to an excitationdirection of the stator 101 while a straight line 107 passing throughthe stepped parts 94 e, 94 f is set to a position tilted at aninstallation angle θ₇ relative to the straight line 27.

A stator having the stepped parts 94 e, 94 f are disposed at a positionto set the installation angle θ₇ to 0 degrees by superimposing thestraight line 107 passing through the stepped parts 94 e, 94 f shown inFIG. 19 on the straight line 27 so that the stepped parts 94 e, 94 f arepositioned to conform to the connections 31 c, 31 d made of alow-permeability material or nonmagnetic material, is already known.

In the case of such a known stator, an initial phase angle caused by theaction of only the stepped parts of the rotor is substantially about 45degrees as mentioned above, and the addition of action of connections inthe same manner as a pair of recesses renders the initial phase angle ofthe rotor determined by composition of vectors (refer to θ₁ in FIG. 22)to reduce to 30 to 40 degrees.

It is preferable that the initial phase angle ranges from 50 to 60degrees, as mentioned above, to achieve lowering of power consumption,but with the conventional construction having the stepped parts whichconform to the positions of the connections can not achieve suchlowering power consumption.

However, according to the stator 101 shown in FIG. 19, since thestraight line 107 passing through the stepped parts 94 e, 94 f arepositioned at a position where it is rotated counterclockwise (positivedirection) at installation angle θ₇ relative to the straight line 27passing through the centers of the paired connections 31 c, 31 d,thereby obtaining the initial phase angle ranging from 50 to 70 degreesthat is effective for achieving lowering of power consumption of thestep motor.

The stator 101 also can obtain the preferable maximum holding torque andthe initial phase angle of the rotor by composition of vectors acting onthe rotor by the paired connections 31 c, 31 d and the paired steppedparts 94 e, 94 f.

Eleventh Embodiment: FIG. 20

A two-pole step motor for a timepiece according to an eleventhembodiment of the invention is described now with reference to FIG. 20.

FIG. 20 is a plan view showing the construction of a stator of thetwo-pole step motor for a timepiece according to the eleventh embodimentof the invention like FIG. 18, and the components corresponding to FIG.18 are depicted by the same reference numerals.

A stator 111 used in the two-pole step motor for a timepiece accordingto the eleventh embodiment of the invention has two kinds of holdingtorque setting means, namely, an oval type means and a notched typemeans which are different in system.

That is, a rotor hole 112 provided in the stator 111 is formed in nottrue circular shape but in an oval shape such as an egg shape, anellipse, an oblong shape, or the like, and the rotor hole 112 per sefunctions as one of the holding torque setting means.

A pair of recesses 45 a, 45 b are formed on the inner periphery of therotor hole 112 that serve as notched type holding torque setting means.

The rotor hole 112 is not true circular as set forth above. Accordingly,when the rotor hole 112 is true circular, the rotor disposed inside therotor hole 112 is not fixed at its stopping position when the step motoris not operated but when the rotor hole 112 is oval, a holding torquefor stopping the rotor is established. Accordingly, the oval rotor hole112 functions as one of holding torque setting means.

With the stator 111, a straight line 64 passing through the centers ofthe recesses 45 a, 45 b has an installation angle θ₅ relative to astraight line 27 orthogonal to an excitation direction while a long axle113 of the oval rotor hole 112 has an installation angle θ₈ relative tothe straight line 27.

With the step motor using the stator 111, the holding torque and initialphase angle are set by composition of a vector corresponding to holdingtorque established by a pair of recesses 45 a, 45 b and a vectorcorresponding to a holding torque established by the oval shape of therotor hole 112.

FIG. 20 shows exaggeratingly an oval shape of the rotor hole 112, butthe actual dimensions are such that a difference between the long axleand the short axle is about 40 μm relative to the diameter of the rotorhole 112 of 1700 μm on average.

Twelfth Embodiment: FIG. 21

A two-pole step motor for a timepiece according to a twelfth embodimentof the invention is described now with reference to FIG. 21.

FIG. 21 is a plan view showing the construction of a stator of thetwo-pole step motor for a timepiece according to the twelfth embodimentof the invention like FIG. 20, and the components corresponding to FIGS.19 and 20 are depicted by the same reference numerals.

A stator 121 used in the two-pole step motor for a timepiece accordingto the twelfth embodiment of the invention has a construction of thecombination of connections 31 c, 31 d made of a low-permeabilitymaterial or nonmagnetic material and an oval type rotor hole 112.

That is, first and second stator parts 121 a, 121 b made of ahigh-permeability material respectively are integrated with each otherby welding through the intermediary of connections 31 c, 31 d eachhaving a width Wb, thereby making up the stator 121. The oval rotor hole112 is formed in the stator 121.

With the stator 121, although a straight line passing through thecenters of the connections 31 c, 31 d is superimposed on a straight line27 orthogonal to an excitation direction, an oval long axle 113 ispositioned at an installation angle θ₈ relative to the straight line 27.

With the stator 121, the connections 31 c, 31 d have the same effect asa pair of recesses as set forth in other embodiments of the invention,and the stator 121 is considered, in a wider sense, as a kind ofcombination of an oval type and a notched type holding torque settingmeans.

Accordingly, the holding torque and initial phase angle of the rotor areset by composition of a vector corresponding to the holding torqueestablished by the pair of connections 31 c, 31 d and a vectorcorresponding to a holding torque established by the oval rotor hole112.

Other Modifications

Although the two-pole step motor for a timepiece according to variousembodiments of the invention has been described hereinbefore, therecesses are formed inside of the rotor hole serving as the holdingtorque setting means among the embodiments of the invention.Protuberances may be formed inside the rotor hole instead of therecesses, and they may function as holding torque setting means.

If such protuberances are formed, a holding torque is determined by eachwidth and each height of the protuberances while it is determined byeach width (Wc in FIG. 7) of the recesses in the case of the recesses.

However, when protuberances are formed inside the rotor hole, adispersion is prone to occur in a holding torque because particularlyeach height of the protuberances is strongly susceptible to precisionwith which to process the stator. Accordingly, when manufacturing astator by press working, the employment of recesses is preferable forsetting a holding torque with precision compared with the employment ofthe protuberances in view of the precision of dimensions of molds.

When the holding torque setting means is formed of recesses, theinstallation angle of the holding torque setting means and the directionof the magnetic poles of a rotor magnet when it is in a standstill aredifferent from each other. However, when the holding torque settingmeans is formed of protuberances, the installation angle of the holdingtorque setting means and the direction of the magnetic poles of a rotormagnet when it is in a standstill conform to each other.

If a plurality of protuberances serving as holding torque setting means

If a plurality of protuberances serving as holding torque setting meansare formed inside the rotor hole, electromechanical coupling constantincreases in proportion to the sum of each width of the protuberancessuppose that each height of the protuberances is fixed.

If the electromechanical coupling constant increase, it is possible toobtain the same holding torque by smaller current as evident from anexpression showing a relation between an electromechanical couplingconstant (nΦ) and a driving torque. As a result, a power consumption ofthe two-pole step motor for a timepiece can be reduced.

The expression showing the relation between the electromechanicalcoupling constant (nΦ) and the driving torque is represented by

Td=n·Φ·i·sin(θ+θ_(i))

where Td is a driving torque, n is the number of turns of a coil, Φ is amaximum magnetic flux interlinking with a coil, i is a current caused toflow through the coil, θ is displacement angle from a standstillposition of the rotor, and θ_(i) is an initial phase angle.

The relation between the sum of each width of a plurality ofprotuberances and the electromechanical coupling constant is representedby concrete numeric values.

A table represented hereunder shows at what ratio the electromechanicalcoupling constant increases as the sum of each width of theprotuberances increase suppose that an electromechanical couplingconstant (nΦ) is 1 in the case of formation of only the recesses servingas holding torque setting means.

Sum of each Width of Increase Ratio of Electromechanical Protuberances(μm) Coupling Constant 0 1.00 400 1.02 600 1.03 800 1.04 1200 1.06

As mentioned above, if the protuberance are formed inside the rotor holeserving as holding torque setting means, an electromechanical couplingconstant can increase when enlarging each width of the protuberances,thereby obtaining an intended driving torque with a small current.Accordingly, a power consumption of the two-pole step motor for atimepiece can be reduced.

Industrial Applicability

As mentioned in detail above, the two-pole step motor for a timepiece ofthe invention can achieve lowering of power consumption by reducing acurrent caused to flow through a coil wound around a magnetic core andcan be manufactured with ease, and hence it is expected to be utilizedin a wider range as a motor for operating hands of an analog electronictimepiece such as a wrist watch or a table clock.

What is claimed is:
 1. A The two-pole step motor for a timepiece comprising: a rotor including a rotor magnet and a rotor axle; a stator comprising a high-permeability material, having a rotor hole in which the rotor is installed; and a field coil for excitation, including a magnetic core comprising a high-permeability material around which a conductor is wound, and opposite ends of which are magnetically bonded to opposite ends of the stator; wherein the stator comprises a plurality of holding torque setting means disposed on inner periphery of the rotor hole at installation angles differing in a direction of the inner periphery; and wherein at least one of the plurality of the holding torque setting means is formed in shape asymmetrical with respect to a center of the rotor hole, wherein said holding torque setting means formed in a shape asymmetrical with respect to the center of the rotor hole comprises a pair of recesses facing each other, formed on the inner periphery of the rotor hole, on opposite sides of the center of the rotor hole.
 2. A The two-pole step motor for a timepiece comprising: a rotor including a rotor magnet and a rotor axle; a stator comprising a high-permeability material, having a rotor hole in which the rotor is installed; and a field coil for excitation including a magnetic core comprising a high-permeability material around which a conductor is wound, and opposite ends of which are magnetically bonded to opposite ends of the stator; wherein the stator comprises a plurality of holding torque setting means disposed on inner periphery of the rotor hole at installation angles differing in a direction of the inner periphery; and wherein at least one of the plurality of the holding torque setting means is formed in share asymmetrical with respect to a center of the rotor hole, wherein said holding torque setting means formed in a shape asymmetrical with respect to the center of the rotor hole comprises a pair of protuberances facing each other, formed on the inner periphery of the rotor hole, on opposite sides of the center of the rotor hole.
 3. The two-pole step motor for a timepiece according to claim 2, wherein said pair of recesses are differentiated from each other in at least one of width and depth.
 4. The two-pole step motor for a timepiece according to claim 2, wherein said pair of protuberances are differentiated from each other in at least one of width and depth.
 5. The two-pole step motor for a timepiece according to claim 1, wherein said stator is made up by bonding a first stator part made of a high-permeability material and a second stator part made of a high-permeability material through an intermediary of two connections made of a low-permeability material or a nonmagnetic material, and the connections serve as said holding torque setting means, except one formed in the shape asymmetrical with respect to the center of the rotor hole.
 6. The two-pole step motor for a timepiece according to claim 2, wherein said stator is made up by bonding a first stator part made of a high-permeability material and a second stator part made of a high-permeability material through an intermediary of two connections made of a low-permeability material or a nonmagnetic material, and the connections serve as said holding torque setting means, except one formed in the shape asymmetrical with respect to the center of the rotor hole. 